Voice calls made over the internet, or voice-over-IP (VoIP), have become increasingly popular.
Some VoIP systems comprise IP telephones, such as those belonging to the Cisco Unified IP Phones 7900 Series. Each IP phone is connected to the Internet and is in communication with a communication server, also connected to the Internet, which manages call connections. Typically, a user can log into any one of a number of IP phones in communication with the communication server and make calls. The IP phone the user logs into takes on a personal phone number that belongs to the user, not the phone as in a traditional POTS or PSTN systems. This is called extension mobility.
It is also possible, in some VoIP networks, for a mobile worker to log into a phone in another geographical area serviced by another communications server, but to have this phone behave as if connected to the first mentioned communications server.
On arrival at a new location within the geographical area, the mobile worker logs into the IP phone. Calls to the user's personal phone number are received at the phone the user has logged on to. Phone calls placed by the user are processed as if the user was located at their usual office phone.